Evidence that garlic may inhibit platelet aggregation, increase fibrinolysis, reduce blood pressure, enhance antioxidant activity, and reduce serum lipids suggests that it may have cardioprotective properties. Lipid lowering and blood pressure reduction have been most extensively studied, and we have undertaken a meta-analysis to examine these effects. The objective was to pool the results of comparable randomised controlled clinical trials, which individually have limited statistical power, to obtain a more stable estimate of the effect of garlic treatment.

A computerised literature search was conducted to identify studies. Published reviews, reference lists from clinical trials, and conference abstracts were also examined. Manufacturers of garlic preparations and authors of published reports were approached to identify unpublished studies. Data were extracted independently by two researchers, and the methodological quality of the studies was assessed. The effect of garlic on systolic and diastolic blood pressure was measured as the difference (in mmHg) between the mean change in blood pressure (baseline-final value) in the two groups. The effect of Garlic on serum cholesterol, triglycerides, and high density lipoprotein (HDL) cholesterol was calculated using the same method.

A total of 11 randomised controlled trials that included an examination of the effect of garlic on blood pressure were identified, and 8 were eligible for inclusion (which provided data on 415 subjects). The mean difference in reduction of mean systolic blood pressure between garlic-treated and placebo- treated subjects was -7.7 mmHg (95% Confidence Intervals -4.3, -11.0 mmHg), and for diastolic blood pressure was -5.0 mmHg (95% CI-2.9,-7.1 mmHg). All these studies have utilised the same garlic powder preparation (Kwai, Sapec - Lichtwer Pharma GmbH)

A total of 25 randomised controlled trials that included an examination of the effect of garlic on serum cholesterol were identified, and 16 were eligible for inclusion (which provided data on 952 subjects). The mean difference in reduction of total cholesterol between garlic-treated and placebo- treated subjects was -0.77 mmol/1 (95% CI -0.65, -0.89 mmol/1), and in 8 trials that measured triglycerides was -0.31 mmol/1 (95% CI -0.14, -0.49 mmol/1). Only 4 trials measured HDL cholesterol, and there was a statistically insignificant reduction of -0.04 mmol/1 (95% CI-0.11,0.03 mmol/1).

These results suggest that garlic therapy may reduce both blood pressure and serum lipids, but they should be interpreted with caution because of methodological shortcomings of some of the trials.

Click here for Figure 4 - Effect of Garlic Therapy vs Placebo on Systolic Blood Pressure.
Click here for Figure 5 - Effect of Garlic Therapy vs Placebo on Total Serum Cholesterol.

References
Silagy C, Neil A. Garlic as a lipid lowering agent - a meta-analysis. J Royal Coll Physicians Lond 1994; 28: 39-45
Silagy C, Neil A. A meta-analysis of the effect of garlic on blood pressure. J Hypertension 1994; 12: 463-468